Large scale heat exchanger systems are essentially comprised of a primary system which contains a large number of individual tubes which have fluid circulating through them, and a secondary system which consists of a second fluid surrounding said tubes contained within a housing which enwraps both systems. In large scale heat exchanger systems, and especially in heat exchanger systems utilized in nuclear reactors, an often recognized problem has been the loss of efficiency of the heat exchanger system due to the build-up of products of corrosion, oxidation, sedimentation and comparable chemical reations on the inner walls of the tubes comprising the primary circulation system. More recently, it has been discovered that the secondary system is also plagued with similar problems such as the build-up of scale, oxides and similar products of corrosion on the outer walls of the tubes comprising the primary circulation system and in particular between the tubes and the support structure for the tubes. Solutions to this problem which are relatively non-destructive to the heat exchanger are desired.
Ever since nuclear reactors have been employed for the generation of electrical power, concern has been focused upon the primary heat exchanger system and on the necessity for maintaining the tubes and conduits of the primary circulation system therein free of anything that could adversely affect either the heat exchanging capability of said tubing or the unimpeded flow of fluid through said tubing. At the same time, it was recognized that to a lesser extent, the same concerns affected the secondary system.
In very large sized heat exchangers, and especially those used in conjunction with steam generating nuclear reactors, the primary system usually comprises a large number of individual tubes which have a primary fluid circulated through them. These tubes are placed in a large receptacle containing a secondary fluid. The primary fluid which carries the heat is circulated through the primary tubes in order to transfer the heat to the secondary fluid which is circulated through the receptacle. To maximize the surface available for heat exchange, the primary tube system contains a very very large number of tubes which are bundled spaced apart from each other.
Each of the large number of tubes in said primary system has a relatively small diameter. A principal concern in such systems has been the possibility of occluding and/or restricting the flow of fluid through these relatively small diameter tubes. It is also recognized that any build-up on the interior walls of the conduits or tubes adversely affects the heat exchange properties of the primary system.
In the past, similar concerns have not been directed to the secondary system which, in many cases, is the steam generation system. Therefore, in the secondary system, the principal concerns have been only that there be an adequate supply of fluid in the primary system, and that the opportunity and volume for the generation of steam is made available.
The problem of maintaining the unimpeded flow of fluid through the large number of tubes in the primary system and the efficiency of heat exchange capability of these primary system tubes is one to which a great deal of effort has been devoted. A chemical cleaning process for an entire nuclear power station was described in detail in a paper presented at the 21st Annual Water Conference of the Engineers' Society of Western Pennsylvania on Oct. 26, 1960, by M. F. Obrecht, et al, entitled "Chemical Cleaning of Boiling Water Reactor and Steam Water System at the Dresden Nuclear Power Station."
In recent years, however, a hitherto unknown but disturbing phenomena has been encountered, especially in heat exchange systems of some of the larger nuclear reactors. These utilize tube bundles in the primary system which are retained in alignment by spacer grids and support plates.
In many such systems, the tubing in the primary system was made of a relatively corrosion resistant material such as Inconel. The support structure for the tubing, however, was made of steel. In the elevated temperatures and the less than ideal fluid environment of the heat exchanger, in addition to the normal build-up of scale and other corrosion or oxidation products on the surface of the various components, it has been discovered that the steel support structure, itself, oxidized to magnetite, especially in the areas immediately adjacent the tubing in the primary system.
The support structure is comprised of spacer grids and support plates. The steel support plates, which in many heat exchanger designs are located in the upper portion of the tube bundles, are fabricated with a plurality of perforations or apertures, each to accomodate a tube of the tube bundle and to maintain the tubes adequately spaced and aligned in the secondary chamber during the installation process. Once the tube bundle was fastened in place, in some heat exchanger designs there was no further need for the troublesome support plates, but there was no easy way to remove them.
While the creation of magnetite is not wholly unexpected, the adverse consequences of its creation had not been fully appreciated. Magnetite, which is a ceramic material and is relatively "spongy", occupies a greater spatial volume than the steel which has been oxidized to form the magnetite. As the steel support structure oxidizes to magnetite and the magnetite builds up at the area where the tubing is surrounded by the support plate, the aperture between the support plate and tubing is reduced, and magnetite eventually fills the space between the support plate and the tubing.
As the oxidation process of steel to magnetite continues a phenomena known as "denting" or "pinching" takes place. The tubing in the primary system of the heat exchanger is constricted by the increasing volume of the magnetite, and the tubing can then be damaged and/or cracked. Further, the flow through the tubing can be substantially impeded at the site of the restriction. Eventually, the usefulness of the tube is reduced to virtually nothing and the tube must be capped at its base. When over 25% of these tubes are capped, the heat exchanger can no longer operate properly and a major and very costly repair of the entire heat exchanger unit must be undertaken.
The continued creation of magnetite with its volumetric increase over the steel it has replaced also tends to cause cracking and distortion of the steel support plates themselves. Fittings and other restraints attached to the support plates cannot accommodate this "expansion" process and structural stresses which are capable of exceeding the limits of the structure are generated thereby creating a deformation of the surrounding structure of the heat exchanger.
Experiments have been conducted to determine ways in which the heat exchanger system can be cleaned and the build-up removed. Chemical methods, such as those discussed in the above-identified paper of Obrecht, et al have been considered. Further, pilot scale experiments have been conducted to determine the relative efficiency of various chemical formulations in the "cleaning" process.
It has been found that more or less conventional chemical cleaning methods utilizing more or less accepted chemical cleaning formulations are so slow as to endanger the integrity of the heat exchanger system. That is, the same formulation which dissolves the magnetite and other scale and corrosion products, if left long enough to be effective, also attacks the basic structural elements of the heat exchanger as well. Further, the cleaning process is inhibited, especially in the apertures between the tubing and support plate, if the cleaning fluid cannot be adequately circulated or agitated to continually bring a fresh supply of cleaning fluid to the site to be cleaned.
It has long been known that sonic cleaning is a useful method for the decontamination of critical or precision parts and assemblies. The American Society for the Testing of Materials published, among other things, a special technical publication No. 342 in 1962, entitled "Cleaning and Materials Processing for Electronics and Space Apparatus."
In an article entitled "The Role of Cavitation in Sonic Energy Cleaning," written for that publication by T. J. Bulat, at page 119, the phenomenon of sonic cleaning is discussed at great length. It was suggested by Bulat, for example, that lower frequencies are better for cleaning massive parts and for penetrating interstices. Further, the effects of temperature were reviewed, revealing that in water, efficiency increases with elevated temperature until approximately 170.degree. F. Higher temperatures appear to cause a loss in efficiency. However, it was suggested that optimum temperature ranges are more a function of the cleaning fluid to be utilized or the temperature at which the contaminants are most susceptible to breakdown. As summarized by Bulat, cleaning by sonic cavitation provides a direct and effective mechanical agitation to speed up the soil removal process and, at the same time, maintain a maximum concentration gradient of cleaning chemical at the surface to be cleaned. Further, the energy for cleaning can be focused and directed so that cavitation can be made to occur deep within the interstices of a part or of an assembly with a complicated geometric configuration.
Most early researchers endeavored to utilize sonic energy to keep the interior of the primary tubes free from surface deposits during use. See, for example, the patent to G. A. Worn, et al U.S. Pat. No. 2,664,274. That invention was primarily directed at improving the efficiency of heat exchangers by removing deposits from the interior of the tubing in the primary system and preventing the formation of deposits within said tubing during operation. Similarly, the patent to Bernard Ostrofsky, et al, U.S. Pat. No. 3,295,596, also taught the removal of deposits from the tubes of a heat exchanger while on-stream at elevated temperatures, through the use of a special liquid coupling device which isolated a sonic transducer from the adverse effects of the elevated temperatures in the heat exchanger system.
Yet another approach utilizing sonic energy has been disclosed by Alvin B. Kennedy, Jr., et al. U.S. Pat. No. 4,120,699 which teaches a continuous varying of the frequency or phase relationship of opposing accoustic wave trains which "sweep" over the surfaces of the body to be cleaned. It would seem that the Kennedy method is intended to clean the surfaces and restrict sedimentation. It appears, however, that the methods and apparatus described therein are intended for normal, preventive maintenance, and are not suited by themselves to the problems presently being considered.